1. Field of the Invention
The present invention relates to a radio data transmission system which realizes multi-level amplitude modulation and more particularly to a radio data transmission system comprising a transversal type automatic equalizer which is capable of changing the mode of a weighting circuit control signal.
2. Description of the Related Art
In a radio data transmission system, multi-path interference occurs resulting in fading because the signal waves pass different routes depending on the conditions of air. This fading causes the frequency characteristic of the radio propagation path to change and thereby generates inter-symbol interference due to waveform distortion. Therefore, an automatic equalizer is necessary for eliminating such inter-symbol interference. To provide such an automatic equalizer, a transversal type automatic equalizer which controls signals within the time domain is often employed.
This transversal type automatic equalizer realizes automatic control of the equalizing function by tap control (weighting control) of the weighting circuit thereof. Therefore, efficiency of control is directly related to the control characteristic and it is essential to improve the tap control performance. In addition, it is a recent trend that an integrated circuit is often employed for said transversal type automatic equalizer and it is also expected in line with such tendency to develop a circuit structure which helps reduction in the size of such a tap control system.
A new transversal type automatic equalizer comprises a delay means with taps, such as a shift register which sequentially delays input signals to a delay line, multipliers connected to respective taps, adders which add the outputs of multipliers, discriminators which discriminate levels of output signals of the adders and control means with weighted taps which detect an error signal corresponding to inter-symbol interference, estimate a degree of inter-symbol interference by adding error signals to correlation circuits and controls the multipliers.
The discriminator discriminates the level of added outputs, detects an error based on signal level required at the time of discrimination, detects pulses, which pulses at the time of discrimination, are causing such error, by using correlation circuit and makes the error approach zero by controlling the weighting the coefficient of the multiplier) for the pulses which give inter-symbol interference.
The tap weighting control of the transversal type automatic equalizer is carried out in the following two modes; one is the normal mode for the above controls and the other is the reset mode where equalization is not carried out by fixing the weighting of the main taps to the reference values and setting the weighting of the taps other than the main taps to zero. These modes are switched in accordance with the equalizing condition.
The mode switching control has been conducted using an accummulated value of erroneous discrimination of frame signals inserted for each frame of transmission data, in place of the predetermined value.
However, in the mode switching explained above, the signal which is used, for controlling the mode switching cannot be extracted if the accummulated pulses do not reach a considerably large value and accordingly it is impossible to execute fine control for mode switching and improvement in tap weighting control performance becomes insufficient. Moreover, such mode switching control is followed by a disadvantage that the necessity of performing frame synchronization requires a relatively large tap weighting control circuit.
Therefore, it is strategically considered to use a discriminator which discriminates levels of multivalue signals in more detail than the number of bits of the transmission data and outputs the digital signals in such a number of bits as is larger than the number of bits of data in order to always monitor the line condition with a simplified additional circuit, while generating a pseudo error signal using the extra bits and executing tap weighting control depending on the frequency of such pseudo error signals. However, the generating frequency of the pseudo error signal is considerably influenced by the equalizing condition because the digital values are discriminated in more detail than the transmission data. Accordingly, it is probable that the generating frequency of the pseudo error signal becomes low and it is misjudged that the line condition is improved and the reset mode is switched to the normal mode. In this case the transversal type automatic equalizer produces abnormal values of tap weighting due to deterioration of line conditions in the reset mode attaining the no equalizing condition. Since the line condition is actually not improved, the equalizer is returned again to the reset mode. Namely, the equalizer produces unstable control conditions by being switched between the normal mode and the reset mode and thereby a problem that judgement of line quality frequently changes arises.
Moreover, the transversal type automatic equalizer shows distortion resistance frequency characteristics (signature) which are largely different when fading gradually becomes large, causing a bit error rate to increase, or when a bit error rate in the line OFF condition or the equivalent gradually becomes smaller because fading gradually decreases, and shows a hysteresis characteristic. Namely, even in a receiving condition of the same bit error rate, if the bit error rate changes to a large value from a small value, the automatic equalization is carried out but, on the contrary, when the bit error rate changes to a small value from a large value, the automatic equalization is not carried out in a certain range.